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Counting Teeth on a Comb

Accurate determination of optical frequencies with high precision usually relies on a colossal experimental setup involving the microwave input of a cesium atomic clock, subsequent frequency doubling and tripling through a chain of stable lasers, all the while comparing and locking frequencies along the chain. The optical frequency of interest is then determined by comparison to the output from this chain.

To bridge the six orders of magnitude between the microwave and optical frequencies in a single step, Diddams et al. used a train of femtosecond pulses generated at a rate of 100 MHz to form an optical comb that serves as a precise frequency grid for making relative comparisons (see also Jones et al., Reports, 28 April, p. 635). To get an absolute map of the frequency spectrum, they couple part of this comb into a microstructured optic fiber, which then generates a broad continuum that covers most of the visible spectrum. Overlaying the precise grid onto this spectrum then allows them to determine other unknown frequencies by counting the number of teeth on the comb between the frequency of interest and its second harmonic with the accuracy and precision otherwise reserved for national laboratories.—ISO